Chemical evolution of the Paleoproterozoic Rooiberg Group Kaapvaal Craton South Africa : new insights into the formation of a silicic large igneous province (SLIP)

Abstract:

With an estimated erupted volume of 300,000 km3 and an areal extent of more than 200,000
km2, the Paleoproterozoic (2.06 Ga) silicic volcanic rocks of the Rooiberg Group (Kaapvaal
Craton) in northern South Africa forms one of the largest and to the same time oldest silicic
large igneous provinces (SLIPs) known. This large volume of rocks can be sub-divided into
four formations: the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formations.
The results of this study show that a clear chemostratigraphy (by using major elements such
as TiO2, SiO2, Na2O, K2O, P2O2, MgO, and Fe2O3) can be established in the area north of
Loskop Dam, dividing the rocks of the study area into the Damwal, Kwaggasnek and
Schrikkloof formations. The studied rocks are characterized by aphanitic lavas bearing
amygdales, spherulitic textures and flow-bands with some sedimentary and pyroclastic
interbeds. The dacites could mainly be described as high-Mg felsites (HMF), whereas the
rhyolites could be described as low-Mg felsites (LMF).
The negative Eu anomaly, Nb and Ta values of the upper part of the Rooiberg Group range
between 5.38-24.2 and 0.45-1.86 ppm, respectively, similar to crustal compositions.
Furthermore, Nb/Ta values range from 10.91-14.83 (also similar to typical crustal
compositions) while few samples from the Damwal Formation exhibit higher values of
15.13-16.02, similar to mantle-derived compositions. Tectonic discriminant diagrams show
that the rocks used in this study evolved from fractional crystallization of a mafic liquid
although all samples plot in fields with crustal signatures. Plot of ƐNd and 87Sr/86Sr show a
mantle-derived origin for the upper part of the Rooiberg Group. However, ƐNd values of the
upper part of the Rooiberg Group range between ~-10 to ~-6, typical of crustal composition
or continental basalts formed in the crust.
From the results, the Rooiberg Group exhibit both mantle (as observed in the Dullstroom and
lower Damwal formations) and crustal signatures as exhibited by the Kwaggasnek and
Schrikkloof formations. This is interprested as a result of the interaction of the thick crust and a shallow mantle source within the Bushveld Province during magmatism. Furthermore,
similarities in geochemical signatures between the Rooiberg Group and selected SLIPs
around the world suggest a similar origin for SLIPs by fractional crystallization of a mafic
melt and melted (or assimilated) crustal material.